1. Field of the Invention
The present invention relates to metal-ceramic combinations and more particularly, to mechanical elements composed of a metallic member and a ceramic member combined with the metallic member.
2. Related Art Statement
Ceramics such as zirconia, silicon nitride, silicon carbide and the like are superior in mechanical strength, heat resistance and abrasion resistance, so that they have drawn attention as materials for structural components to be used in high temperature environments or anti-abrasion parts such as gas-turbine engine parts, diesel engine parts, or the like. However, ceramics are relatively inferior, since generally hard and brittle, to metallic materials in molding and finishing properties. Further, it is difficult to form mechanical elements such as engine parts, from a ceramic material alone, owing to its poor toughness, and therefore, ceramics are generally used in a form of composite structure wherein a metallic member is combined with a ceramic member.
As mechanical composite structures comprising a metallic member and ceramic member in metal-ceramic combinations to be used for engine parts, there are known, for example, a structure as shown in FIG. 10 of the attached drawings, wherein a protrusion is provided en bloc on base 1 of ceramic member, e.g. a revolving shaft of turbine wheel, forming an easement portion on the base and around the root of the protrusion, having its cross-sectional area gradually increasing towards the base. The protrusion is combined with metallic member 2, e.g. a revolving shaft of compressor wheel, by shrink fitting a metallic cylindrical collar 15 on and around the peripheral surfaces of the protrusion and the metallic member (Japanese Patent Application Laid-open No. 57-20,061). Another known structure is shown in FIG. 11, wherein protrusion 4 of a ceramic member, e.g. a revolving shaft of turbine wheel, is fitted tightly into recession 3 provided at an end of metallic member 2, e.g. a revolving shaft of compressor wheel (U.S. Pat. No. 3,666,302).
However, those conventional combination structures have such a shortcoming as follows.
Since the edge of the joint portion of the metallic member is so close to the easement portion of the protrusion of the ceramic member, a large stress concentration develops in the easement. Namely, when a bending load is applied to a metal-ceramic combination having a conventional combination structure as shown in FIG. 10, a stress concentration caused by bending is overlapped on a stress-concentrated portion resulting from shrink fitting, thereby developing into a large stress concentration at the easement portion where the structure will eventually break.